This invention relates to a process for manufacturing aluminum alloy sheets. More particularly, the present invention is directed to aluminum alloy sheets suitable for press forming of auto body panels, air cleaners, oil tanks and other like products which require superior strength and formability.
In general, cold rolled steel sheets have been used for press forming of auto body panels or the like. Recently, however, there has been a great demand for aluminum alloy sheets instead of the cold rolled steel sheets. This substitution results in lighter auto bodies, which in turn improves the fuel consumption thereof.
Conventional aluminum alloy sheets having strength and formability, include O stock of Al-Mg alloy 5052 which consists essentially of a chromium alloy containing 2.5 wt. % of Al and 0.25 wt. % of Mg, 0 stock of Al-Mg alloy 5182 which consists essentially of a manganese alloy containing 4.5 wt. % of Al and 0.35 wt. % of Mg and T4 stock of Al-Cu alloy 2036 which consists essentially of a magnesium alloy containing 2.6 wt. % of Al, 0.25 wt. % of Cu and 0.45 wt. % of Mn.
Of the above mentioned alloy sheets, only the Al-Mg alloy sheets have both excellent deep drawing formability and strength. They are often used for deep drawing press-formed products such as inner members.
The prior art Al-Mg alloy sheets for press forming are normally manufactured by a process which includes forming slabs for rolling, homogenizing, hot rolling, cold rolling and final annealing. Additionally, an intermediate annealing step may be included prior to the cold rolling step. In cases requiring flat sheets, a straightening step is often carried out by either a tension leveler, a roller leveler, skin pass rolling or like means after the annealing step.
Conventional Al-Mg alloy sheets manufactured by the above process have superior formability when compared against other aluminum alloy sheets. However, they have inferior formability as compared to cold rolled steel sheets. Therefore, there is such a problem as the Al-Mg alloy sheet is easily cracked at the time of press forming, in comparison with the cold rolled steel sheet. Further, since the Al-Mg alloy sheets have inferior strength as compared to the cold rolled steel sheets, it is difficult to make the Al-Mg alloy sheets thinner. Thus, the overall goal of making lighter auto bodies cannot be obtained.
It is known that elongation of the Al-Mg alloy sheets can be substantially improved in proportion to the Mg content therein. Therefore, prior art methods for producing Al-Mg alloy sheets with improved elongation have attempted to increase the Mg content above that of the conventional Al-Mg alloy (2.5 to 5.0 wt. % of Mg).
For example, Japanese Patent Laid-open No. 4-147936 discloses an aluminum alloy sheet containing 4 to 8 wt. % of Mg, 0.05 to 0.7 wt. % of Cu, 0.01 to 0.3 wt. % of Mn and 0.002 to 0.01 wt. % of Be and having grain diameters in the range of 30 to 100 .mu.m.
As a result of the high Mg content the Al-Mg alloy sheet has a high elongation percentage. Furthermore, since elongation is highly correlative with stretch forming formability, bending formability and flanging formability or the like, these properties are also improved due to the high Mg content.
However, conventional Al-Mg alloy sheets having high Mg content have the following disadvantages.
One drawback is that conventional Al-Mg alloy sheets having high Mg content is inferior in deep drawing formability to cold rolled steel sheets. In particular, when press forming is done under poor lubrication conditions as in the case of press forming of auto parts, the Al-Mg alloy sheets having high Mg content are easily cracked. This degrades productivity.
Although the strength of the conventional Al-Mg alloy sheets having high Mg content is greater than that of other aluminum alloy sheets, the strength is still inferior to that of the cold rolled steel sheets. Therefore, the conventional Al-Mg alloy sheets having with high Mg content cannot be made as thin as required to lighten the weight of auto bodies.
The present inventors have examined the above-mentioned problems of the conventional Al-Mg alloy sheets having high Mg content in detail. As a result, they have found that the higher the strength of a material, the better the deep drawing formability of an aluminum alloy sheet. Further, that an alloy sheet obtained by finely recrystallizing an Al-Mg alloy sheet with properly dispersed intermetallic compounds containing Cr has extremely high strength and has also excellent deep drawing formability.